{"id":"CVE-2026-98299","published":"2026-10-06T09:18:20.697","lastModified":"2026-10-06T09:18:20.697","description":"In the Linux kernel, the following vulnerability has been resolved:\n\ntcp: do not let tcp_rmem be set below 4096\n\nWe can hit a division by zero crash in tcp_rcvbuf_grow()\nand tcp_rcv_space_adjust():\n\ndivide error: 0000 [#1] PREEMPT SMP\nRIP: 0010:tcp_rcvbuf_grow+0x187/0x450 net/ipv4/tcp_input.c:939\n...\ngrow = div_u64(((u64)rcvwin << 1) * (newval - oldval), oldval);\n\nThe division uses oldval = tp->rcvq_space.space as divisor.\nWhen tp->rcvq_space.space is zero, this leads to a divide-by-zero\nexception.\n\ntp->rcvq_space.space is initialized in tcp_init_buffer_space():\n    tp->rcvq_space.space = min3(tp->rcv_ssthresh, tp->rcv_wnd,\n                                (u32)TCP_INIT_CWND * tp->advmss);\n\nIf tcp_rmem[1] is configured to very small values (such as 1),\nsk->sk_rcvbuf is initialized to 1. Then tcp_full_space(sk), which\ncomputes (sk->sk_rcvbuf * scaling_ratio) >> 8, truncates to 0.\nThis sets tp->window_clamp = 0, tp->rcv_ssthresh = 0, and\ntp->rcvq_space.space = 0. Later, when data arrives and DRS is invoked,\ntcp_rcvbuf_grow() divides by oldval == 0.\n\nBack in 2015, commit b1cb59cf2efe (\"net: sysctl_net_core: check SNDBUF\nand RCVBUF for min length\") ensured that net.core.rmem_default and\nnet.core.rmem_max cannot be set below SOCK_MIN_RCVBUF. Similarly,\nSO_RCVBUF setsockopt enforces max_t(int, val * 2, SOCK_MIN_RCVBUF).\n\nHowever, net.ipv4.tcp_rmem still had .extra1 = SYSCTL_ONE, allowing\narbitrarily small values.\n\nBecause SOCK_MIN_RCVBUF depends on sizeof(struct sk_buff) and cacheline\nalignment, its value varies across architectures and configuration options.\nUsing a fixed constant of 4096 ensures a predictable, architecture-\nindependent lower bound that is safely above SOCK_MIN_RCVBUF everywhere\nand matches the documented 4K default.\n\nFix this by setting tcp_rmem.extra1 to 4096 and updating the documentation.","cvssScore":null,"cvssSeverity":null,"cvssVector":null,"cwes":[],"vendors":[],"products":[],"references":[{"url":"https://git.kernel.org/stable/c/1c4bb940c3bb4325bb88ac1b7eaf5faaa39e7d63","tags":[]},{"url":"https://git.kernel.org/stable/c/60df201dbb19a1bf6478b56d100f7ae1fe90e46a","tags":[]},{"url":"https://git.kernel.org/stable/c/67b83c15bed9eeeb8d1a6dbf88a5f79525970173","tags":[]},{"url":"https://git.kernel.org/stable/c/7898bda1ebc198f5559b301a96dd5398edd4d4d3","tags":[]},{"url":"https://git.kernel.org/stable/c/83a945a529d6e002dd7339c532288a931f463dba","tags":[]},{"url":"https://git.kernel.org/stable/c/879907631b9e70eedb62d76461b29afa106ebe7a","tags":[]},{"url":"https://git.kernel.org/stable/c/8e32532d0fa3191ecf9524b0e6bafa0832e712ba","tags":[]},{"url":"https://git.kernel.org/stable/c/9a4f49bf8d2da4e52e904f60c29cca317bab9b3a","tags":[]}],"exploitRefs":[],"hasPoc":false,"ai":null}